1. Field of Invention
This invention relates to light emitting diodes. In particular, this invention is directed to a method and apparatus that improves device stability of a light emitting device pixel by limiting dark spot propagation.
2. Description of Related Art
Dark spot propagation is the propagation of non-operative, i.e., dark, areas in an organic layer, e.g., an electron transport layer, of a light emitting device. Dark spot propagation destroys the emission capability of light emitting diodes in image displays and light-emitting bars used for printing.
Although dark spot propagation effects all types of light emitting devices, polymeric and small-molecule based organic light emitting diodes (OLEDs) suffer an increased susceptibility to dark spot propagation. Dark spots result from defects, such as, for example, physical defects resulting during fabrication. These physical defects can occur because of extraneous unwanted material disposed during formation of the constituent layers of the OLED structure and are on the order of 1 micrometer in diameter or smaller. These dark spots spread from the cathode region of the light emitting diode into regions that carry current.
FIG. 1 illustrates the fabricated layers of a conventional OLED structure 100 including a substrate 105 made of, for example, glass, an anode 110 made of, for example, indium tin oxide (ITO), a hole injecting layer 120, a hole transmission layer 130 made of, for example, N, Nxe2x80x2-diphenyl-N, N-bis(3-methylphyenyl)1,1xe2x80x2-biphenyl-4,4xe2x80x2diamine (TPD), an electron transmission layer 140 made of, for example, tri(8-hyrdoxyquinoline) aluminum (Alq3), and a cathode layer 150 made of, for example, a Magnesium Silver (MgAg) alloy. The Mg cathode provides an appropriate workfunction for efficient electron injection. As shown in FIG. 1, a physical defect 200 on the substrate 100 translates into a discontinuity in the cathode layer 150. This discontinuity gives rise to a dark spot, i.e., a non-emitting area in a pixel because the dark spot is not structured properly to emit light.
During fabrication, measures can be taken to minimize the number of dark spots. However, once a dark spot is produced during fabrication, the dark spot can spread to adjacent pixels. This propagation of the dark spots reduces the light emitting capability of light emitting devices. A finite density of dark spot nuclei is deemed inevitable in any realistic diode array process unless exposure to all unwanted defects can be eliminated in the fabrication process. Therefore, it is incredibly difficult to eliminate the dark spot propagation phenomena by eliminating dark spots during fabrication.
The propagation of dark spots is caused by a number of factors. Dark spots grow in the organic layers of the diode due to the presence of moisture. However, the susceptibility of a device to dark spot propagation appears to depend on the material used, such as for example, the organic layers used in an OLED. As mentioned above, dark spot propagation significantly affects the cathode contact of an OLED.
FIG. 2 shows an optical micrograph of an OLED with dark spot propagation. The light areas are those areas where the active region is still emitting light. The dark areas are those areas that are affected by dark spot propagation and no longer emit light.
Because it is believed that moisture is involved in the degradation of organic layers, hermetic packaging of OLEDs has been used to slow the propagation of dark spot defects in the organic layers by prohibiting the presence of moisture. However, all hermetic packaging will leak in moisture at a finite rate, leading to an eventual permeation of the package with moisture.
Another solution for dark spot propagation is to increase the cathode thickness to better cover any physical defects formed in the organic layer. However, increasing the cathode thickness increases the costs and time associated with fabrication and may not be a significant deterrent to dark spot propagation.
Alternatively, additional material can be deposited between the bottom-most OLED layer and the substrate. One such additional material, for example, is PANI (Polyanaline), which is electrically conducting and conformal to the surface, so that the surface becomes smoother. However, depositing of this additional material also increases the costs and time associated with fabrication.
In the extreme case, dark spot propagation causes the cathode of an OLED to actually delaminate, i.e., separate, from the organic layer disposed below the cathode. Delamination actually allows even more moisture to degrade the organic layer further compounding the degradation. FIG. 3 shows a planar view of an OLED showing the edge of the light emitting area as viewed from the cathode side. In FIG. 3, the current flow is normal to the paper. The effect of dark spot propagation is that the ordinarily shiny cathode metal is blistered in the light emitting portion or area. The light emitting portion of the diode is defined by the crossed portions of the cathode made of MgAg and anode stripes made of indium tin oxide (ITO). That is, blistering and delamination occur only in the regions that carry current through the cathode-organic layer interface, as illustrated in FIG. 3. Where no current flows, no blistering occurs. Additionally, the blistering stops where the light emitting area stops.
The unblistered cathode metal outside the active region of the diode in FIG. 3 transports current parallel to, but not across, the diode interface, i.e., the interface between the cathode and the electron transport layer. Therefore, the dark spot propagation mechanism appears to be stimulated by the passage of current from the cathode into the organic layers.
This invention provides a mechanism including a method and apparatus for deterring dark spot propagation.
This invention deters dark spot propagation by dividing the active emitting area of each light emitting device into sub-regions that are separated by a dielectric barrier. This division prevents spreading of one dark spot affected sub-region to adjacent current carrying sub-regions. A side benefit of the division is improved out-coupling of light from the active emitting area of the light emitting device because the morphology resulting from the incorporation of the dielectric barrier can be configured to increase the amount of externally radiated, and thereby collectable, portion of the generated light.
In a preferred embodiment of the invention, lithographic patterning is used to create a layered structure with a dielectric barrier that is patterned onto the anode in order to divide the anode into electrically isolated sub-regions.